4. SL numbers. The second pair of digits (third and fourth digits) designate a specific location in a
PR. SL numbers are assigned to switches in each PR. The SL in the tactical environment serves the
same purpose as the telephone exchange number in the commercial system.
a. As a planner, you must consider designating all your combat units with unique SL numbers.
Assign a different SL number to the brigade main, alternate, and supply area. By using this method,
combat units can move throughout the combat zone and only the PR digits of the telephone number
must be changed.
b. A system planner must also be aware of and consider unique switches such as the AN/TTC-30
used by the Air Force. This switch cannot access every PR available. Therefore, special considerations
must be made when planning an interface with these switches.
5. Directory numbers. The third element of the seven-digit telephone number is the last three-digit
directory number or subscriber's number. Knowing what the different directory number forms represent
is important for planning and engineering a telephone network. The numbers are assigned to subscribers
who are identified by title and located within a CP. The numbers can be used also to identify activities
within the CP area, such as the G3 operations, combat support, and combat service support units.
6. Dialing. To place a call from one automatic switch to another, or to a manual switchboard, the
entire seven-digit number must be dialed. If the call is to a local subscriber connected to the same
automatic switchboard, only the last three digits need to be dialed. However, if the called number is
serviced by a manual switchboard, all seven digits must be dialed, even if the called party is a local
subscriber connected to the same automatic switchboard.
7. Fixed directory service. The fixed directory service is a feature whereby roving subscribers or units
are given a fixed number. This constant seven-digit number is used regardless of the subscribers or unit
location within a given area code.
Directory services. The telephone directory, as a minimum, should include the following:
a. A complete listing of all elements within the theater, corps, and division. This is to include all
combat service and combat service support units.
b. Instructions on dialing throughout the network, to include how to access a subscriber service by a
manual switchboard. In a fully automated switch network, the telephone directory should include the
seven-digit tactical automatic switching addresses for all the switches.
c. The telephone directory should include a list of all the emergency phone numbers, to include
telephone repair and the operator information.